Such flow passages are known in many cases. For example, U.S. Pat. No. 5,203,673 discloses such a device for controlling and adjusting radial gaps between the tips of rotor blades and the guide ring which lies opposite these, and is part of the passage wall. In this case, it is provided that in the conical flow passage the guide ring is axially displaceable for adjusting radial gaps. For axial displacement of the guide ring, three hydraulic cylinders, the pistons of which can move parallel to the machine axis of the turbine, are screwed in the stator blade carrier and distributed over the circumference. In conjunction with the conical gap between the wall surface of the guide ring, which delimits the flow path, and the inclined tips of the rotor blades, which correspond thereto, the gap dimension, or the radial distance between wall surface and blade tips, can be adjusted by means of the axial displacement. Resetting is carried out by means of helical springs which move the guide ring back into the original position. At the same time, provision is made for the use of a radial gap measuring system, with which the radial gap can be measured at one point. Depending upon the measured gap dimension, the guide ring is then axially positioned so that a smallest possible gap dimension is achieved while avoiding brushing of the blade tip against the wall surface. However, the use of a plurality of hydraulic cylinders is disadvantageous since some of them can fail. This would lead to skewing of the guide ring in the event of adjustment. A further disadvantage of the device is the rather selective initiation -provided at only three points-of the adjusting force by means of the hydraulic cylinders which is required for axial displacement of the guide ring. Each cylinder therefore has to be able to transmit a comparatively large portion of the overall adjusting force, which necessitates space-consuming cylinders.